This invention relates to fuel cells and, more particularly, to fuel cells incorporating polymeric ion exchange membranes for proton transport between electrodes. This invention is the result of a contract with the U.S. Department of Energy (Contract No. W-7405-ENG-36).
Fuel cells operate to convert chemical energy to electrical energy. In one embodiment, a fuel, hydrogen H.sub.2, is oxidized at the anode to form protons H.sup.30, EQU H.sub.2 =2H.sup.+ +2e.sup.+
and the protons traverse an ion exchange membrane to react with oxygen, EQU O.sub.2 +4H.sup.+ +4e.sup.-=2 H.sub.2 O.
One class of polymer electrolyte membrane (PEM) fuel cells uses a solid polymer membrane formed from an ion exchange polymer, such as polyperfluorosulfonic acid, e.g. Nafion, a DuPont Company product. Ion transport is along pathways of ionic networks established by the anionic (sulfonic acid anion) groups that exist within the polymer. Water is required around the ionic sites in the polymer to form conductive pathways for proton transport.
Such solid polymer membranes, however, become non-conductive when the polymer drys excessively and are not then useful for ion transport in fuel cells. The membranes are subject to moisture removal by evaporation due to heat generated in the chemical reaction and current transport (i.e., i.sup.2 R type losses) and from electroosmotic transport from hydrogen water compounds, e.g. "hydronium ions" H(H.sub.2 O).sup.+, which transport water from the anode to the cathode. The electroosmotic "drag" is believed to transport one or two water molecules with each proton. Excess water is then available on the cathode side of the membrane from both the chemical reaction and the electroosmotic transport effects. There is some diffusion of excess water back from the cathode to the anode, but this is not always sufficient to prevent excessive membrane drying under high current operating conditions.
In one approach to maintaining adequate moisture in the membrane, an external humidifier is included in fuel cell system designs for introducing water as steam or a fine mist in the hydrogen gas fuel stream to the anode. However, as seen in FIG. 1, even with a saturated gas, a dehydrated gas is obtained after only a short traverse along a fuel cell channel. The back diffusion of water does provide some anode rehumidification, but even with substantial diffusivity this is inadequate to prevent excessive membrane drying at high current density values. The quantity of moisture carried by a saturated gas can only be increased by increasing the flow of gas, but this requires a recirculating gas system with recirculation pumps and some means of venting impurities which tend to build in the unused gas of the system. Liquid water could be introduced at the anode, but the liquid tends to flood the anode and restrict access of the fuel gas to reaction sites on the anode for current generation.
In conjunction with removal of moisture from the membrane and the final oxidation reaction, water accumulates on the cathode side of the membrane and must be removed promptly to maintain oxygen access to the reaction sites adjacent the membrane. An external recirculation system can also be provided for the oxygen to remove water and reaction heat from the oxygen stream prior to reintroducing the oxygen into the fuel cell. Such a system requires relatively pure reactant gases to minimize impurity buildup and the system must still be vented at intervals to remove trace impurities which accumulate.
In one attempt to resolve these problems, U.S. Pat. 4,769,297, issued Sept. 6, 1988, to Reiser et al., teaches the use of porous electrodes separated by a hydrophilic plate to transport excess water from a cathode flow field to an adjacent fuel cell anode flow field for moistening an adjacent solid polymer membrane. The hydrophilic plate adds volume to the active fuel cell region. Further, the porous plates enable oxygen transport to an adjacent hydrogen gas stream where the resulting hydrogen depletion reduces the overall cell operating efficiency. Evaporation of the water into the hydrogen gas stream is taught to cool the power generation section such that a separate cooling system is not required. However, a two-phase flow appears to be required over at least a portion of the hydrogen flow field with concomitant gas access restriction to reaction sites. There is no discussion about evenly distributing the moisture over the membrane flow field. Also, water transport is cumulative across the stacked fuel cells and the water content cannot be controlled to regulate the humidifying and temperature control effects of the water transport.
These and other problems of the prior art are addressed by the present invention and a PEM fuel cell is provided with internal flow fields that are dedicated to water transport for hydrogen humidification or oxygen dehumidification.
Accordingly, it an object of the present invention to provide a PEM fuel cell with an internal anode humidification flow field for maintaining a moist conductive membrane.
Another object of the invention is to provide a PEM fuel cell with an internal evaporation flow field surface for cooling the cell.
One other object of the invention is to provide a PEM fuel cell having an internal cathode flow field for water removal from the oxygen stream.
Yet another object of the invention is to provide a PEM fuel cell having a flow field sequential with a reaction flow field for use in regulating the humidification and cooling of the hydrogen and oxygen gas streams.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.